It has been commonplace practice to employ drum wound fibres in the test environment. This involves use of bulky and costly material, and it is difficult and/or cumbersome to vary parameters and to reproduce all the conditions likely to arise in system use.
Erstwhile, and because of past scarcity of suitable fibres, it has also been practice to employ a pulse shuttling technique. Accordingly, a fixed length of fiber is interposed between reflectors and a long length fiber simulated by applying an input pulse and selecting a delayed output pulse, corresponding to a predetermined multiple of transits of the single fiber.